Switching regulators typically switch at frequencies from 100 KHz-5 MHz. Small current loops that are created at such high speeds can generate significant magnetic fields. If the switches are formed in an integrated circuit (IC), the current loops may occur entirely within the IC or partially internal and partially external to the IC. If a loop conducts an initial transient current due to the power switch closing or a synchronous rectifier switch closing, the relatively high di/dt results in a high magnetic field that may create electromagnetic interference (EMI).
FIG. 1A illustrates a prior art current loop 10 in a switching power supply, where the entire circuit forming the loop (e.g., transistors, wires/traces, capacitors, parasitics, etc.) adds to the magnetic field, causing EMI. The switches 12 and 14 are shown as MOSFETs, but may be any other type of switch, such as bipolar transistors. The switches 12 and 14 are shown as n-channel MOSFETs, but CMOS transistors may be used, depending on the application. The switches' parasitic capacitances 16 and 18 are shown.
In the example, it is assumed that the switches 12 and 14 are synchronously switched to alternately couple a power supply voltage (Vcc) at terminal 1 to the output terminal 2 and then couple a ground voltage at terminal 3 to the output terminal 2. An inductor (not shown) may be coupled to terminal 2 as part of an output circuit 17 in a buck regulator. In one example of a regulator, a PWM controller IC 19 is connected to the gates of the two switches 12 and 14 for controlling the switch duty cycle to output a regulated voltage Vout or regulated current. The duty cycle is controlled to cause a feedback voltage Vfb, proportional to the output voltage of the regulator, to match a reference voltage. One of the switches is referred to as the power switch and the other is the synchronous rectifier, which essentially acts as a low voltage drop diode. The switches 12 and 14 are not both on at the same time to avoid a short circuit.
The regulator may be any type (e.g., buck, boost, flyback, etc), and the particular connection of the switches 12 and 14 to the inductor is determined by the type of regulator. The present invention may be employed in any type of switching regulator, and certain examples are given to illustrate the concepts of the invention as applied to a standard buck regulator switching configuration.
A conventional bypass capacitor 20 is shown in FIG. 1A connected between terminal 1 and terminal 3. Bypass capacitors are typically used in prior art switching circuits. If we assume terminal 1 is connected to Vcc and terminal 3 is ground, the bypass capacitor 20 smooths transients when the switches 12 and 14 open and close. For example, when the regulator controller closes the switch 12 to charge the inductor, there is a surge of current by the charged capacitor 20 being connected to the inductor via terminal 2. This helps offset any droop in the Vcc caused by the surge of current and smooths the switching transient. The capacitor 20 forms part of the current loop around terminals 2, 3, and 1. The parasitic capacitances 16 and 18 of the switches 12 and 14 are also part of the current loop, and the parasitic capacitor 18 charges when the switch 14 is turned off and the switch 12 is turned on.
As seen, there is a very fast and high current pulse (having a large di/dt) being generated through the loop 10 at the switching frequency. This pulse generates EMI, which may interfere with or create distortion in nearby electrical circuits.
The switches 12 and 14 may be within an IC 21 and may be part of the PWM controller IC 19.
FIG. 1B is another representation of the circuit of FIG. 1A, which illustrates the transistor switch locations on a printed circuit board (PCB) or in an IC package. The terminals 1, 2, and 3 may be the connection nodes for the pins extending from the IC package. The current loop 10 shows the transient current flowing in a counter-clockwise direction, causing the direction of the magnetic field to be upwards, represented by a dot. The magnetic field emanates in all directions in a certain pattern.
What is needed is a technique for reducing the overall magnetic field generated by a switching regulator in order to reduce EMI.